Science · 6 pages · 12 min
How a microwave heats water in food
The oven's waves tug polar water molecules until that motion becomes heat.
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From the opening
A microwave oven heats food mainly by driving polar water molecules back and forth with an alternating electric field. The jostling becomes heat inside the food, rather than heat soaking inward from a hot wall of the box.
It does not work by making the air in the box hot the way an ordinary oven does. The walls and the air can stay comparatively cool while a wet potato becomes too hot to hold.
The mechanism is more particular than the slogan. A water molecule has a positive end and a negative end, so the microwave field twists it, it bumps its neighbors, and the organized motion degrades into random heat. Household ovens use a frequency that water absorbs and that regulators set aside for this job.
Follow it one step further. Ice absorbs these waves less willingly than liquid water, because the molecules are locked in a crystal and cannot turn freely. A frozen core can lag behind a thawed wet spot, which then heats fast and leaves the piece uneven.
The case worth keeping in mind is specific. A bowl of soup heats while a dry ceramic plate, holding little water, stays milder than the food on it. The turntable exists because standing waves make hotter and cooler zones, and moving the food averages those zones.
That case has a boundary. This is dielectric heating of polar molecules, especially water, not a universal warming of whatever is in the box. Metals reflect the waves and can spark where charge crowds at an edge, which is a different failure, and truly dry food warms poorly.
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Inside
- Polar molecules driven by the field
- Why ice lags behind liquid water
- Hot spots and the turntable
- Why the door mesh holds waves in
- Where the idea stops applying
- A distinction worth keeping
Sources
- Dielectric heating of polar water molecules
- Standing waves inside a microwave cavity